The field of the invention is that of systems for receiving complex (phase and amplitude) modulated digital signals using a temporal equalizer. The invention applies to receivers for PDM-M modulated signals and to QAM-M type receivers used in digital radio systems, for example, where the value of M is 4, 8, 16 or greater.
Receiving systems of this kind are described in "Digital Communications" by John G. PROAKIS, McGraw-Hill.TM..
FIG. 1 shows part of a system for receiving a signal with multiple phases. A received signal x(t) at the intermediate frequency or in the base band is applied to each first input of two mixers 10, 11 the second inputs of which receive local oscillator signals in phase quadrature. These signals are from a 90.degree. phase-shifter 12 receiving a sinusoidal signal from a local oscillator 13. The output signals of the mixers 10, 11 constitute two channels I and Q in phase quadrature and are fed to lowpass filters 14, 15 driving analog-digital converters 18, 19 supplying samples Z, W of the quadrature signals. The samples Z and W are applied to a set 20 of adaptive filters receiving correction parameters from a calculating device 24. The set 20 of filters and the device 24 constitute a temporal equalizer. The set 20 of filters includes four transversal filter branches and drives a signal regenerator 23 supplying regenerated signals X and Y.
The function of a temporal equalizer 20, 24 is to correct all types of linear distortion and therefore to reduce the effect of residual amplitude modulation or group delay time distortion of the transmission system. It is able to eliminate intersymbol interference and fading to minimize the transmission error rate.
The output signals x and y of the temporal equalizer 20, 24 are also fed to a carrier recovery device 21 controlling the local oscillator 13 and to a timing recovery device 22 controlling the analog-digital converters 18 and 19 so that the signals they receive are sampled at the times at which the eye diagram is widest open. The timing recovery device 22 supplies a clock signal H to these converters for this purpose.
The regenerator 23 supplies error signals e.sub.x and e.sub.y to a device 24 for calculating multiplier coefficients supplied to the temporal equalizer 20 (see below) and also to the carrier recovery device 21. The regenerated signals X and Y are also fed to the calculating device 24.
FIG. 2 shows one of the four branches of a set 20 of filters with five coefficients in this example. The input signal E, corresponding to the signal Z or W in FIG. 1, is applied to successive time-delay lines which delay each sample by a time period Ts corresponding to the symbol time. Each sample is multiplied by a coefficient C.sub.ES-2 to C.sub.ES+2 supplied to it by the coefficient calculator device 24 (quadrature estimator) and the results of the various multiplications are summed by a summing device 25 whose output signal F corresponds to the estimated sample x or y which is used to optimize the coefficients C.sub.ES-2 through C.sub.ES+2. The coefficient C.sub.ES0 is known as the center coefficient of the filter.
The regenerator 23 converts the received signal into information words and quantifies the distortion of the received signal (x and y) relative to the ideal signal (X and Y). For example, a pointing error e.sub.0 on a sample x.sub.0 has the following effect:
for pointing anterior to the optimum time:
* e.sub.0 &lt;0 if x.sub.0 -x.sub.-1 &gt;0 and x.sub.+1 -x.sub.0 &gt;0 PA0 * e.sub.0 &gt;0 if x.sub.0 -x.sub.-1 &lt;0 and x.sub.+1 -x.sub.0 &lt;0 PA0 * e.sub.0 &gt;0 if x.sub.0 -x.sub.-1 &gt;0 and x.sub.+1 -x.sub.0 &gt;0 PA0 * e.sub.0 &lt;0 if x.sub.0 -x.sub.-1 &lt;0 and x.sub.+1 -x.sub.0 &lt;0 PA0 and .gamma.(t-.tau.)=sgn(X.sub.t-.tau.).multidot.sgn(e.sub.x) or sgn(x).multidot.sgn(ex.sub.t+.tau.), PA0 where sgn( ) corresponds to the sign function and .tau. to the time at which the estimate is made. .tau. can be equal to 1, for example. The pointing estimator eliminates the error e.sub..alpha..beta. to render the impulse response symmetrical. PA0 at least one peripheral control loop, the estimator of which uses at least one of the correction parameters supplied to the temporal equalizer, the peripheral control loop having an operating range separate from that of the temporal equalizer; PA0 temporal discriminator means detecting inactive areas of the estimator of each peripheral control loop to enable operation of the temporal equalizer in such a way as to prevent interaction between the temporal equalizer and the peripheral control loop. PA0 and .gamma.(t-.tau.)=sgn(x.sub.t-.tau.).multidot.sgn(e.sub.x) or sgn(x).multidot.sgn(ex.sub.t+.tau.), PA0 where sgn( ) is the sign function, in which case the temporal discriminator means detect the condition: EQU .gamma.(t+.tau.)=.gamma.(t-.tau.)
for pointing posterior to the optimum time:
where x.sub.-1 and x.sub.+1 respectively correspond to the samples preceding and following the sample x.sub.0.
The purpose of the temporal equalizer is to correct distortion between the I and Q channels at the times decisions are taken, i.e. at each symbol time Ts, set by the clock H of the timing recovery device 22, by seeking an orthogonal relationship between the signals x and y.
FIG. 3 shows the set 20 of filters from FIG. 1 in which each branch 30 through 33 can comprise a device as shown in FIG. 2. The respective output signals of the branches 30, 33 and 31, 32 are summed at 34 and 35 to produce the signals x and y.
The orthogonal relationship between x and y is obtained by calculating a matrix of correlation between Z and W and between x and y. This correlation matrix features the center correction parameters C.sub.Zx0, C.sub.Wy0, C.sub.Zy0 and C.sub.Wx0 which are respectively the center parameters of the filters 30, 31, 32 and 33: ##EQU1##
If pointing of the eye of the signal is not effected at the optimum time by the timing recovery device 22, i.e. when the eye is widest open, the estimated errors e.sub.x and e.sub.y are non-null and the coefficient calculator device 24 attempts to apply a correction by modifying the multiplier parameters of the filter 20 to correct the pointing error. The temporal equalizer and the timing recovery device therefore act in conjunction.
The timing recovery device 22 can operate in various ways and one robust analog synchronization method entails detecting zero crossings of the signals x and y. This detection occurs on the upstream side of quadrature signal sampling and is therefore not protected against thermal drift or bias due to distortion of the received signal x(t) in the case of multipath propagation.
Other known estimators are based on the symmetry of the impulse response to within .+-. Ts. This type of estimator is described in the article "Timing Recovery in Digital Synchronous Data Receivers", IEEE Transactions on Communications, vol.COM-24 n.degree. 5, May 1976, by Kurt H. Mueller and Markus Muller. It would be desirable if it could be used in the context of the present invention.
However, this type of estimator, referred to hereinafter as the Mueller and Muller type, is not compatible with a temporal equalizer as it is identical to that used by the temporal equalizer.
The estimator of the temporal equalizer included in the coefficient calculator device 24 modifies the first before phase and first after phase coefficients. These coefficients are referred to hereinafter as C.sub..alpha..beta.-1 and C.sub..alpha..beta.+1 where .alpha. corresponds to Z or W and .beta. to x or y, depending on the branch concerned. These coefficients are denoted C.sub.ES+1 and C.sub.ES-1 in FIG. 2 for the filter shown there. The Mueller and Muller article shows that optimization of pointing of the eye diagram amounts to obtaining symmetry between the first before phase and first after phase coefficients. To this end the following error e.sub..alpha..beta. is calculated, for example: EQU e.sub..alpha..beta. =sgn(x.sub.+1).multidot.sgn(e.sub.x)-sgn(x.sub.-1).multidot.sgn(e.sub.x)
A more general form of the above equation is as follows: EQU e(t)=.gamma.(t+.tau.)-.gamma.(t-.tau.)
where .gamma.(t+.tau.)=sgn(X.sub.t+.tau.).multidot.sgn(e.sub.x) or sgn(x).multidot.sgn(ex.sub.t-.tau.),
Consequently, if a temporal equalizer is associated with a Mueller and Muller type pointing estimator, for example, in parallel loops, the receive system as shown in FIG. 1 is rendered unstable. To use a temporal equalizer and a pointing estimator adapted to render the impulse response symmetrical at the same time, it would be necessary to hold at zero the coefficients C.sub.Zx-1, C.sub.Zx+1, C.sub.Wy-1 and C.sub.Wy+1 of the estimator at zero, the effect of which would be to prevent locking on of the eye diagram pointing. It follows that these two devices are incompatible.
The receive system of FIG. 1 also uses a carrier frequency recovery device 21 which controls the local oscillator 13.
Carrier recovery devices of this kind use known digital estimators. An estimator using the center coefficients C.sub..alpha..beta.0 is used, for example, in the so-called Leclert and Vandamme control loops described in the article "Universal Carrier Recovery Loop for QASK and PSK Signal Sets", by A. LECLERT and P. VANDAMME, IEEE TRANSACTIONS ON COMMUNICATIONS, vol. com-31, n.degree. 1, January 1983, pages 130 to 136.
It would be desirable if a Leclert and Vandamme type estimator could be used for carrier recovery in the context of the present invention.
However, interaction also occurs between the estimator of the temporal equalizer and the carrier recovery estimator if the latter uses the center coefficients C.sub..alpha..beta.0 of the temporal equalizer. It is then necessary to hold the coefficients C.sub.Zy0 and C.sub.Wx0 of the temporal equalizer at zero while the carrier estimator is operating, which prevents quadrature error correction and causes intersymbol distortion.